Conventionally, it has been known to connect an ignition coil device to a spark plug to boost a voltage applied from an external power source. For example, JP2003-243234A (hereinafter referred to as the patent document 1) and JP08-273950A (corresponding to U.S. Pat. No. 5,603,307 and hereinafter referred to as the patent document 2) describe examples of such an ignition coil device. The described ignition coil device has a primary coil connected to a power source, a power transistor that switches on and off of electric power supply from the power source to the primary coil, and a secondary coil that generates a voltage to cause spark discharge.
Further, the patent document 1 describes to connect a resistor for noise reduction in series with a conducting section that electrically connects the secondary coil to the spark plug. The patent document 2 describes to connect a buffer coil in series with a conducting section that electrically connects the secondary coil to the spark plug.
Specifically, in the ignition coil device of the patent document 1, when the electric power supply to the primary coil is switched from an off state to an on state by the power transistor, a high voltage to cause spark discharge is induced in the secondary coil. The voltage is outputted from the secondary coil to the spark plug to cause breakdown between electrodes of the spark plug, thereby to generate the spark discharge. In accordance with such an electric conduction between the electrodes, an electric current instantly flows through the conducting section and respective components of the ignition coil device connected to the conducting section. An instant change of the electric current caused by the spark discharge induces a conduction noise in a component of the ignition coil device. Further, a radiation noise induced by the conduction noise is radiated from the component of the ignition coil device.
The noise reduction resistor of the patent document 1 reduces the instant change of the electric current in the conducting section by electric resistance (impedance). The buffer coil of the patent document 2 reduces the instant change of the electric current in the conducting section by impedance of the inductance. In this way, the noise reduction resistor and the buffer coil are employed to reduce the conduction noise and the radiation noise generated from the component of the ignition coil device.